Having no dog in the fight this. If you look at the map developed as to spawning areas it is not the large number impacted. One never knows about unforeseen consequences which are always around. After that this whole thing started due to desire to keep I-5 from being closed again. The flood that hit them last time was more or less a flash flood and a dam will not effect that at all. Lots of rain / short time frame you got trouble.

Take the Satsop 4 in two days in a row you have a big flood but normal. Now if the 4 in come down back to back such as 8 pm to midnight and then midnight to 4 or 5 am you get a bloody 100 year flood. If the river is already at or near flood stage when this happens you get a 10,000 year flood. At to the mix in the 1930's ES ( he lived in the valley ) said that when that flood happened he had 18 in of snow on the ground and everyone was snowbound when the rain hit blowing like hell. He went to bed and when he got up to stoke the fire at daylight none, all gone. That day the water got in his barn which he had never seen before and has not happened since.